did you set up roaming in your mobile account before leaving the UK?(If a pay monthly customer, you will need to have been with Virgin Mobile for more than 3 months to activate roaming).
Log into your account > click on the "Plan and device" tab.
Go to "Manage my services" > check that "Roaming" is set to ON & "International call barring" is set to OFF.
If they are not set correctly, make the change by moving the "switch" in your account.(Click the other side of the rectangle box).
Virgin advise that any change may take upto 24 hours or longer from abroad to come into effect.
Reboot your phone, then try doing a manual network search.
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